What are quantum effects in gravity from a delocalized quantum source, and how should gravitational physics be described when reference systems are themselves quantum? In this talk, I will address these questions through two complementary developments in linearised quantum gravity.
I will start from presenting two effects arising from delocalised quantum sources that cannot be reproduced by the Newton potential and are independent of graviton emission. The first effect concerns interactions between two generic quantum sources of gravity, such as sources in wide Gaussian states. The second is an additional contribution to the relative phase of a quantum source interacting with a test particle, governed by the commutator between the gravitational field and its canonically conjugate momentum. Observing these two effects would give further quantitative information and stronger evidence, compared to experiments only involving the Newton potential, to show that gravity is nonclassical.
I will then turn to the relational description of matter and geometry using quantum reference fields: sets of dynamical scalar fields whose stress-energy enters the gravitational constraints, and whose quantum states cannot be sharply peaked on definite field configurations. By generalizing the perspective-neutral construction of quantum reference frames, we show that relational, gauge invariant observables admit reduced descriptions in the perspective of each quantum reference field, and we derive the unitary transformations relating them. A relational von Neumann measurement scheme shows how the corresponding observables can be accessed operationally. Together, these developments explore both the observable quantum features of gravity and the role of physical quantum reference systems in defining gravitational observables and measurements.